Animal behavior is both stable and plastic. To these ends, molecules and neural circuits respond to environmental changes, both short-term and long-term pertubations. In some cases, the responses maintain a fixed behavior despite a radically altered external environment, whereas in others the responses underlie an adaptive change. Learning and memory, for example, are an important class of adaptive changes to altered environmental stimuli. This Program Project group is principally focused on temperature as the environmental stimulus of interest, and all five laboratories work on poikliotherm invertebrates. There are three Drosophila groups, one C. elegans group and one group working on the crab stomatogastric ganglion (STG). All are interested in the molecules, neurons, networks and mechansms that underlie stability, sensory responsiveness, and change (learning) in the context of altered environmental stimuli. Although each of the five projects is self-contained, there are strong and important connections between groups: In Project 1, Rosbash is collaborating with Sengupta and asking, are there circadian rhythms in C. elegans and what are the relevant molecules? In this same project, Rosbash is collaborating with Garrity and asking, what are the temperature sensors that govern circadian entrainment by temperature in Drosop/7/7a? Even projects that are not directly collaborative are well-connected, as the STG group (Marder, 4) is asking a question of profound interest to a circadian laboratory (Rosbash, 1): how does network output remain constant despite substantial changes in temperature? This is a classic circadian problem but has never been addressed in the context of non-circadian molecules, cells and circuits. Project 1 also addresses the same overarching question in the traditional circadian context. Both Project 2 and Project 5 are addressing thermotaxis and thermosensory responses, in Drosophila and C. elegans, respectively. Project 5 (S